Could Solar Radiation Pressure Explain ’Oumuamua’s Peculiar Accelaration

5 Pages·2018·0.248 MB·English

by  Shmuel Bialy, Abraham Loeb

DRAFTVERSIONNOVEMBER1,2018
PreprinttypesetusingLATEXstyleemulateapjv.12/16/11
COULDSOLARRADIATIONPRESSUREEXPLAIN‘OUMUAMUA’SPECULIARACCELERATION?
SHMUELBIALY(cid:63)ANDABRAHAMLOEB
HarvardSmithsonianCenterforAstrophysics,60Gardenst.,Cambridge,MA,02138
DraftversionNovember1,2018
ABSTRACT
‘Oumuamua (1I/2017 U1) is the first object of interstellar origin observed in the Solar System. Recently,
Michelietal.(2018)reportedthat‘OumuamuashoweddeviationsfromaKeplerianorbitatahighstatistical
significance. The observed trajectory is best explained by an excess radial acceleration ∆a∝r−2, where r is
8
thedistanceof‘OumuamuafromtheSun. Suchanaccelerationisnaturallyexpectedforcomets,drivenbythe
1
evaporating material. However, recent observational and theoretical studies imply that ‘Oumuamua is not an
0
activecomet. WeexplorethepossibilitythattheexcessaccelerationresultsfromSolarradiationpressure. The
2
requiredmass-to-arearatiois(m/A)≈0.1gcm−2. Forathinsheetthisrequiresawidthof≈0.3−0.9mm.

t Wefindthatalthoughextremelythin,suchanobjectwouldsurviveaninterstellartraveloverGalacticdistances
c
O of∼5kpc,withstandingcollisionswithgasanddust-grainsaswellasstressesfromrotationandtidalforces.
Wediscussthepossibleoriginsofsuchanobjectincludingthepossibilitythatitmightbealightsailofartificial

0 origin. Ourgeneralresultsapplytoanylightprobesdesignedforinterstellartravel.
3 Subjectheadings:ISM:individualobjects(1I/2017U1)–minorplanets,asteroids: individual(1I/2017U1)–
General: extraterrestrialintelligence–Minorplanets,asteroids: general
]
P
E
1. INTRODUCTION If not cometary activity, what can drive the non-
h. OnOctober19,2017,thefirstinterstellarobjectintheSo- gravitationalaccelerationobserved? InthisLetterweexplore
p larSystem,‘Oumuamua(1I/2017U1)wasdiscoveredbythe thepossibilityof‘Oumuamuabeingathinobjectaccelerated
- PAN-STARRS1survey. Ithasahighlyhyperbolictrajectory bySolarradiationpressure,whichwouldnaturallyresultinan
o (witheccentricitye=1.1956±0.0006)andpre-entryveloc- excessacceleration∆a∝r−2. 1 However,forradiationpres-
r
t ity of v ≈26 km s−1 (Meech et al. 2017). Based on the suretobeeffective,themass-to-arearatiomustbeverysmall.
s ∞ In §2 we derive the required mass-to-area ratio (or effective
a surveypropertiesandthesingledetection,Doetal.(2018)es-
[ timatedtheinterstellardensityofobjectslike‘Oumuamuaor width), and find (m/A)≈0.1 g cm−2, corresponding to an
largertoben≈2×1015pc−3,2-8ordersofmagnitudelarger effective thin sheet of width w≈0.3−0.9 mm. We explore
2 the ability of such an unusually thin object to survive inter-
than expected by previous theoretical models (Moro-Martin
v stellartravel,consideringcollisionswithinterstellardustand
et al. 2009). The large variations in its apparent magnitude
0 gas (§3), as well as to withstand the tensile stresses caused
and the non-trivial periodicity of the lightcurve, suggest that
9 by rotation and tidal forces (§4). Finally, in §5 we discuss
‘Oumuamuaisrotatinginanexcitedspinstate(tumblingmo-
4
the possible implications of the unusual requirements on the
1 tion),andhasanextremeaspectratioofatleast5:1(Fraser
shapeof‘Oumuamua.
1 et al. 2018; Drahus et al. 2018), an unprecedented value for
. previously known asteroids and comets in the Solar System.
0 2. ACCELERATIONBYRADIATIONPRESSURE
Belton et al. (2018) have shown that if ‘Oumuamua rotates
1 in its highest rotational energy state, it should be extremely Micheli et al. (2018) had shown that Oumuamua’s experi-
8
oblate(pancake-like). encesanexcessradialacceleration,withtheirbestfitmodel
1
: Recently, Micheli et al. (2018) reported the detection of (cid:16) r (cid:17)n
v non-gravitational acceleration in the motion of ‘Oumuamua, ∆a=a0 (1)
AU
i at a statistical significance of 30σ. Their best-fit to the data
X with n=−2,
is obtainedfor a modelwith a non-constantexcess accelera-
r tionwhichscaleswithdistancefromtheSun,r,as∆a∝r−2, a =(4.92±0.16)×10−4cms−2. (2)
a 0
butotherpower-lawindexvaluesarealsopossible.Theycon-
cludedthattheobservedaccelerationismostlikelytheresult Thevaluefora0isaveragedovertimescalesmuchlongerthan
ofacometaryactivity. Yet,despiteitscloseSolarapproachof ‘Oumuamua’srotationperiod.
r=0.25AU,‘Oumuamuashowsnosignsofaanycometary Anaccelerationofthisformisnaturallyproducedbyradi-
activity, no cometary tail, nor gas emission/absorption lines ationpressure,
L
wereobserved(Meechetal.2017;Knightetal.2017;Jewitt (cid:12)
P=C , (3)
et al. 2017; Ye et al. 2017; Fitzsimmons et al. 2017). From R4πr2c
a theoretical point of view, Rafikov (2018) has shown that
whereL istheSolarluminosity,cisthespeedoflight,and
(cid:12)
if outgassing was responsible for the acceleration (as origi-
C is a coefficient of order unity which depends on the ob-
R
nally proposed by Micheli et al. 2018), then the associated
ject’s composition and geometry. For a sheet perpendicular
outgassing torques would have driven a rapid evolution in
totheSun-objectvectorC =1+ε whereε isthereflectivity.
R
‘Oumuamua’sspin,incompatiblewithobservations.
1Interestingly,asimilarapproachwasadoptedhistoricallyregardingthe
(cid:63)[email protected] anomalousorbitofPhobus(Shklovskii1962).
2
3.1. MomentumTransfer-SlowDown
AnobjectwithacrosssectionalareaAtravelingadistance
LthroughtheISMwouldaccumulateanISMgasmassof,
M =AΣ =1.4m (cid:104)n(cid:105)LA, (7)
gas gas p
whereΣ istheaccumulatedmasscolumndensityofinter-
gas
stellar gas, m is the proton mass, (cid:104)n(cid:105) is the mean proton
p
number density averaged along the object’s trajectory, and
the factor 1.4 accounts for the contribution of helium to the
mass density of the ISM. For trajectories that span Galactic
distancesthecontributionoftheSolarSystemtotheaccumu-
lated column is negligible. The contribution of accumulated
dusttothemomentumtransferisalsonegligible,becausethe
typicaldust-to-gasmassratiointheGalaxyis≈1/100.
Once M approaches the object’s mass, m, the momen-
ISM
tum of the traveling object will decrease by a significant
amount. TherequirementM /m(cid:28)1translatesintoamax-
ISM
FIG.1.— Themaximumallowedtraveldistancethroughtheinterstellar imumallowedvalueontheobject’smass-to-arearatio,giving
medium(ISM),asafunctionof(m/A). Theblueandredlinesarelimita-
tionsobtainedbyslow-downduetogasaccumulation,andvaporizationby (cid:16)m(cid:17)
=Σ =1.4m (cid:104)n(cid:105)L (8)
dust-collisions,respectively. TheplottedresultsareforameanISMproton A min,p gas p
densityof(cid:104)n(cid:105)∼1cm−3. Alllinesscaleas1/(cid:104)n(cid:105). Thedashedmagentaline
isourconstrainton‘Oumuamuabasedonitsexcessacceleration. TheSolar =7.2×10−3(cid:104)n(cid:105) L gcm−2.
0 0
Galactrocentricdistanceisalsoindicated.
Inthelastequalitywenormalizedtothetypicalvalues(cid:104)n(cid:105) =
0
Foraperfectreflectorε=1,andCR=2,whereasforaperfect (cid:104)n(cid:105)/(100cm−3)andL =L/(100kpc).
0
absorberε =0andC =1.
R Givenamass-to-arearatio,themaximumtraveldistanceis
ForanobjectofmassmandareaAtheaccelerationwould
be 1 (cid:16)m(cid:17)
L = (9)
max,p
PA (cid:18) L (cid:19)(cid:18)A(cid:19) 1.4mp(cid:104)n(cid:105) A
(cid:12)
a= m = 4πr2c m CR (4) =14(cid:104)n(cid:105)−1(cid:16)m(cid:17) kpc.
(cid:16) r (cid:17)−2(cid:18) m/A (cid:19)−1 0 A −1
=4.6×10−5 C cms−2. In the second equation we denoted (m/A) ≡
AU gcm−2 R −1
(m/A)/(10−1gcm−2). Figure 1 shows the results from
ComparingEq.(1)and(4)wefindthattherequirementonthe Eq. (9) as a function of (m/A). The dashed vertical line
mass-to-arearatiois indicates our constraint on (m/A) for ‘Oumuamua (Eq. 5).
(cid:16)m(cid:17) Evidently, ‘Oumuamua can travel Galactic distances before
=(9.3±0.3)×10−2CRgcm−2. (5) encounteringappreciableslow-down.
A
For a planer body with mass density ρ, this translates into a 3.2. EnergyTransfer-CollisionswithDust-Grains
requirementonthebody’swidth
Collisions with dust grains at high velocities will induce
m craterformationbymeltingandevaporationofthetargetma-
w = =(9.3±0.3)×10−2ρ−1C cm, (6)
Aρ 0 R terial. Since the typical time between dust collisions is long
comparedtothesolidificationtime,anymoltenmaterialwill
where ρ =ρ/(100 gcm−3). Typically, ρ ≈1−3, giving solidify before the next collision occurs, and thus will only
0 0
athinsheetof0.3–0.9mmwidth. Othergeometriesarealso cause a deformation of the object’s surface material, not re-
possible, and are discussed in §5. The force exerted by the ductioninmass. Ontheotherhand,atomsvaporizedthrough
Solarwindonasolidsurfaceisnegligiblecomparedtothatof collisionscanescapeandthuscauseamassablation.
theSolarradiationfieldandisneglectedhereafter. Wewouldliketoestimatetheminimummass-to-arearatio
Theobservedmagnitudeof‘Oumuamuaconstrainsitsarea required for the object to not lose significant fraction of its
to be A ≈ 8×106α−1 cm2, where α is the albedo (Jewitt massupondust-graincollisions. Letmd bethecollidingdust-
et al. 2017). This corresponds to an effective radius, R ≡ grain mass, and φ the fraction of the kinetic energy that is
√ eff
A/π =16α−1/2 meters. Forourestimationofthemass-to- converted into vaporization of the object’s body. The total
numberofvaporizedatomspercollisionisthen,
arearatio,thisareatranslatesintoamassofm≈740(C /α)
R
kg. m m v2
v d
N = =φ , (10)
v
3. MAXIMUMDISTANCEFORINTERSTELLARTRAVEL m¯ 2Uv
Nextweexploretheimplicationsofimpactswithinterstel- wherem isthemassofvaporizedmaterialintheobject,m¯ is
v
lar dust-grains and gas particles, in terms of momentum and the mean atomic mass of the object, andU is the vaporiza-
v
energy transfer. We obtain general requirements for the ob- tionenergy.Althoughhighlysimplistic,thisanalysiscaptures
ject’s mass-to-area ratio, or alternatively, for the maximum the results of the detailed theoretical model of Tielens et al.
interstellar distance that can be traveled before it encounters (1994)andtheempiricaldatafromOkeefe&Ahrens(1977).
appreciableslow-downorevaporation. Agoodmatchtothenumericalresultsisobtainedforφ =0.2.
3
OveradistanceLtherewillbemanycollisions. Adopting
TABLE1
theconservativeassumptionthatforeachcollisionalltheva-
TENSILESTRENGTHS
porizedmaterialescapestotheISM,wecanaccountforallthe
collisionsalongapath-length,L,byreplacingm inEq.(10) material tensilestrength(dynecm−2)
d
withthetotalaccumulateddustmass, 67P/Churyumov-Gerasimenko1 10-50
Meteorites2 (1−5)×107
Md =ΣdustA=ΣgasϕdgA=1.4mp(cid:104)n(cid:105)LϕdgA, (11) Iron 3×107
Diamond 2×1010
whereΣ isthedustcolumndensityandφ isthedust-to-
dust dg Silicon(monocrystalline) 7×1010
gasmassratio. Thisgives,
1Attreeetal.(2018)
m 1.4m (cid:104)n(cid:105)Lϕ Av2 2Petrovic(2001)
v p dg
=φ . (12)
m¯ 2U
v Theminimalmass-to-arearatiobelowwhichhalfoftheob-
Requiringthatthetotalvaporizedmassnotexceedhalfofthe ject’smasswillbesputteredis,
object’smass, weobtainaconstraintontheminimummass- (cid:16)m(cid:17)
to-arearatiooftheobject, =m¯(cid:104)n(cid:105)LYtot (17)
A min,s
(cid:16)m(cid:17) =1.4m (cid:104)n(cid:105)L(cid:18)φϕdgm¯v2(cid:19) (13) =6.2×10−5m¯12(cid:104)n(cid:105)0L0Y−3gcm−2.
p
A min,m Uv In the second equality we normalized toY =10−3, corre-
=3.1×10−4(cid:104)n(cid:105) L (cid:18)ϕ−2m¯12v226(cid:19) gcm−2. spondingtokineticenergiesE≈30−100etoVt,corresponding
0 0 U tov≈40−70kms−1. Forlowerspeeds,asthatof‘Oumua-
4
mua, the yield is even lower, further decreasing the value of
Herewedefinedthenormalizedparameters,ϕ−2=ϕdg/10−2, (m/A)min,s.Athigherspeeds,theyieldincreasesbuttypically
m¯12 =m¯/(12mp), as appropriate for carbon based materials remainsbelow0.01(Tielensetal.1994),thusatanyvelocity,
(e.g.,graphiteordiamond);U4=U/(4eV),asappropriatefor vaporizationandslow-downremainthedominatingprocesses
typicalvaporizationenergies(e.g.,forgraphite,Uv=4.2eV); limitingthealloweddistanceanobjectcantravelthroughthe
andv =v/(26kms−1),thevelocityatinfinityof‘Oumua- ISM.
26
mua. For a given mass-to-area ratio, the maximum allowed Cosmic-rays are expected to cause even less damage. Al-
distancebeforesignificantevaporationis thoughtheirenergydensityiscomparabletothatoftheISM
1 (cid:18) U (cid:19)(cid:16)m(cid:17) gas,theydepositonlyaverysmallfractionoftheirenergyas
L = m (14) theypenetratethroughthethinobject.
max,d 1.4m (cid:104)n(cid:105) φϕ m¯v2 A
p dg
(cid:18) U (cid:19)(cid:16)m(cid:17) 4. TENSILESTRESSES
=330(cid:104)n(cid:105)−01 ϕ−2m¯412v226 A −1 kpc. forAcetshiinfiotbsjteecntscilaensbtreetnogrtnhaipsanrottbsyucffienctireinfutlgyasltfroorncge.sToyrptiidcaall
Forourconstrainedvalueforthemass-to-arearatio,‘Oumua- valuesforthetensilestrengthsofvariousmaterialsareshown
mua can travel through the entire galaxy before a significant in Table 1. Next, we calculate whether centrifugal or tidal
fraction of its mass is evaporated. Evaporation becomes im- forcescandestroy‘Oumuamua.
portantathigherspeeds.ComparingEqs.(8)and(13)wefind
thatonlyforspeedsabove 4.1. Rotation
(cid:115) Omuamua’s lightcurve shows periodic modulations on an
2Um orderof6-8hours. Ignoringthetumblingmotion,letusesti-
v = (15)
crit m¯ϕφ matethetensilestressoriginatingfromthecentrifugalforce.
The largest stress is produced if the object is elongated such
=130(cid:18) U4 (cid:19)1/2 kms−1, that the longest dimension is perpendicular to the rotation
ϕ m¯ axis. Wedenotethisdimensionasd. Consideringtheobject
−2 12
asmadeoftwohalves,eachlocatedwithacenterofmassata
vaporizationdominatesoverslow-down. distanced/4fromtherotationaxis,andigonringselfgravity,
aradialforceofmagnitude,
3.3. EnergyTransfer-CollisionswithGasParticles
1 d
When an object travels at a high speed, collisions with F = mΩ2 , (18)
atomsintheISMcanpotentiallytransfersufficientenergyto 2 4
producesputtering.Thisprocesswasstudiedinthecontextof willbeexertedoneachhalf. Theassociatedtensilestressis,
dustgrainsinhotshocks(Tielensetal.1994). Foranobject
1
traveling at a velocity v, over a distance L through the ISM, P = ρd2Ω2 (19)
rot
thetotalnumberofsputteredparticlesis, 4
ms =0.25ρ0d42Ω2−4dynecm−2,
N = =2AL(cid:104)n(cid:105)Y , (16)
s tot
m¯ whered ≡d/(104 cm),Ω ≡Ω/(10−4 s−1). Thisismuch
4 −4
where Ytot = ∑iYixi is the total sputtering yield (which de- smallerthantypicaltensilestrengthsofnormalmaterials,and
pends on the kinetic energy), summed over collisions with evenofthatofthecomet67P/Churyumov-Gerasimenko(see
differentspecies(i.e.,H,He,andmetals),andx istheabun- Table 1). Thus, even when self-gravity is ignored, ‘Oumua-
i
danceofthecollidingspeciesrelativetohydrogen. muacaneasilywithstanditscentrifugalforce.
4
4.2. TidalForces estimatefor‘Oumuamua. Ifradiationpressureistheacceler-
ating force, then ‘Oumuamua represents a new class of thin
Thetidalforcewillbemaximalifthelongdimensionofthe
interstellar material, either produced naturally,through a yet
object is parallel to the Sun-object vector. Again, modeling
unknownprocessintheISMorinproto-planetarydisks,orof
theobjectasconsistingoftwohalvesasin§4.1,thedifference
anartificialorigin.
inthegravitationalforceexperiencedbythefarandnearends
Considering an artificial origin, one possibility is that
oftheobjectis,
‘Oumuamuaisalightsail,floatingininterstellarspaceasade-
1 GM m
(cid:12)
δF ≈ d , (20) brisfromanadvancedtechnologicalequipment(Loeb2018).
4 r3
Lightsails with similar dimensions have been designed and
where r is the distance of the center of mass from the Sun. constructed by our own civilization, including the IKAROS
Theassociatedtensilestress, project and the Starshot Initiative2. The lightsail technol-
1 GM ogy might be abundantly used for transportation of cargos
Ptid≈ 4ρd2 r3(cid:12) (21) betweenplanets(Guillochon&Loeb2015)orbetweenstars
(Lingam&Loeb2017). Intheformercase, dynamicalejec-
(cid:16) r (cid:17)−3
=9.9×10−7ρ d2 dynecm−2. tion from a planetary System could result in space debris of
0 4 AU equipment that is not operational any more 3 (Loeb 2018),
Evenatperihelion(r=0.25AU),thetensilestressisnegligi- and is floating at the characteristic speed of stars relative
ble. to each other in the Solar neighborhood. This would ac-
The critical distance below which tidal forces dominate count for the various anomalies of ‘Oumuamua, such as the
overcentrifugalis unusual geometry inferred from its lightcurve (Meech et al.
2017; Fraser et al. 2018; Drahus et al. 2018; Belton et al.
(cid:18)GM(cid:19)1/3 −2/3(cid:18) M (cid:19)1/3 2018), its low thermal emission, suggesting high reflectivity
R = =3.4Ω R . (22)
tid Ω2 −4 M (cid:12) (Trilling et al. 2018), and its deviation from a Keplerian or-
(cid:12)
bit (Micheli et al. 2018) without any sign of a cometary tail
Thus,unless‘Oumuamuaencounteredanextremelycloseap-
(Meech et al. 2017; Knight et al. 2017; Jewitt et al. 2017;
proachtoastarinitspast,itisunlikelythattidalforcesplayed
Ye et al. 2017; Fitzsimmons et al. 2017) or spin-up torques
anysignificantrole.
(Rafikov 2018). Although ‘Oumuamua has a red surface
5. SUMMARYANDDISCUSSION color,similartoorganic-richsurfacesofSolar-Systemcomets
and D-type asteroids (Meech et al. 2017), this does not con-
We have shown that the observed non-gravitational accel-
tradicttheartificialscenario,sinceirrespectiveoftheobject’s
eration of ‘Oumuamua, may be explained by Solar radia-
composition,asittravelsthroughtheISMitssurfacewillbe
tion pressure. This requires a small mass-to-area ratio for
covered by a layer of interstellar dust, which is itself com-
‘Oumuamua of (m/A)≈0.1 g cm−3. For a planar geome-
posedoforganic-richmaterials(Draine2003).
try and typical mass densities of 1–3 g cm−3 this gives an Alternatively, a more exotic scenario is that ‘Oumuamua
effective width of only 0.9–0.3 mm, respectively. For a ma- may be a fully operational probe sent intentionally to Earth
terialwithlowermassdensity,theinferredeffectivewidthis vicinitybyanaliencivilization.BasedonthePAN-STARRS1
proportionally larger. We find that although very thin, such surveycharacteristics,andassumingnaturaloriginsfollowing
an object can travel over galactic distances, maintaining its random trajectories, Do et al. (2018) derived that the inter-
momentumandwithstandingcollisionaldestructionbydust- stellarnumberdensityof‘Oumuamua-likeobjectsshouldbe
grains and gas, as well as centrifugal and tidal forces. For extremelyhigh,∼2×1015pc−3,equivalentto∼1015ejected
‘Oumuamua, the limiting factor is the slow-down by accu-
planetisimals per star, and a factor of 100 to 108 larger than
mulated ISM mass, which limits its maximal travel distance
predicted by theoretical models (Moro-Martin et al. 2009).
to ∼10 kpc (for a mean ISM particle density of ∼1 cm−3).
This discrepancy is readily solved if ‘Oumuamua does not
Our inferred thin geometry is consistent with studies of its
followarandomtrajectorybutisratheratargetedprobe. In-
tumbling motion. In particular, Belton et al. (2018) inferred
terestingly, ‘Oumuamua’s entry velocity is found to be ex-
that‘Oumuamuaislikelytobeanextremelyoblatespheroid
tremelyclosetothevelocityoftheLocalStandardofRest,in
(pancake)assumingthatitisexcitedbyexternaltorquestoits
akinematicregionthatisoccupiedbylessthan1to500stars
highestenergystate.
(Mamajek2017).
Whileourscenariomaynaturallyexplainsthepeculiarac-
Sinceitistoolatetoimage‘Oumuamuawithexistingtele-
celerationof‘Oumuamua,itopensupthequestionwhatkind
scopesorchaseitwithchemicalrockets(Seligman&Laugh-
ofobjectmighthavesuchasmallmass-to-arearatio? Theob-
lin 2018), its likely origin and mechanical properties could
servations are not sufficiently sensitive to provide a resolved
only be deciphered by searching for other objects of its type
imageof‘Oumuamua,andonecanonlyspeculateonitspos-
in the future. In addition to the vast unbound population,
siblegeometryandnature.Althoughperiodicvariationsinthe
thousands of interstellar ‘Oumuamua-like space-debris are
apparentmagnitudeareobserved,therearestilltoomanyde-
expected to be trapped at any given time in the Solar Sys-
grees of freedom (e.g., observing angle, non-uniform reflec-
temthroughgravitationalinteractionwithJupiterandtheSun
tively, etc.) to definitely constrain the geometry. The geom-
(Lingam&Loeb2018). Deepwide-areasurveysofthetype
etryshouldnotnecessarilybethatofaplanarsheet,butmay
acquireothershapes,e.g.,involvingacurvedsheet,ahollow
cone or ellipsoidal, etc. Depending on the geometry our es- 2 A list of books and papers on lightsails is provided in
http://breakthroughinitiatives.org/research/3. The IKAROS project is
timatedvalueforthemass-to-arearatiowillchange(through
discussedinhttp://global.jaxa.jp/projects/sat/ikaros/.
CRinEq.5),butthecorrectionistypicallyoforderunity. 3Notethat‘Oumuamuawasfoundnottoshowanyradioemissiondown
Known Solar System objects, like asteroids and comets toafractionofthepowerofacellphonetransmission(Enriquezetal.2018;
have mass-to-area ratios orders of magnitude larger than our Tingayetal.2018;Harpetal.2018).
5
expectedwiththeLargeSynopticSurveyTelescope(LSST)4 WethankManasviLingam,PaulDuffell,QuanzhiYe,and
willbeparticularlypowerfulinsearchingforadditionalmem- ananonymousrefereeforhelpfulcomments. Thisworkwas
bersof‘Oumuamua’spopulationofobjects. supported in part by a grant from the Breakthrough Prize
AsurveyforlightsailsastechnosiganuresintheSolarSys- Foundation.
temiswarranted,irrespectiveofwhether‘Oumuamuaisone
ofthem.
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